An electric variable cam timing (VCT) system is developed as a device that optimizes, for example, a valve open/close timing of an air-intake value for supplying air to an engine and an exhaust valve for exhausting an exhaust gas from the engine, based on a variable and continuous control of a cam phase according to a state of driving of a vehicle, such as an engine rotation number, an accelerator opening, and the like, for the purpose of appropriate engine control and torque output.
The prior art in the field, e.g., a technique disclosed in a patent document, (Japanese Patent Laid-Open No. 2008-118823) (patent document 1) listed below, is about a motor rotation control, in which a target motor rotation speed and an actual motor rotation speed are compared with each other with reference to a data table, and an apply voltage to a motor drive circuit is calculated based on such comparison. When the calculated apply voltage takes a negative value, a rotation direction of a motor is reversed from a current/actual rotation direction. More practically, such a technique is used to accurately control a wiper reverse position based on a torque reversing at a transition moment of the calculated apply voltage transitioning from a positive value to the negative value, i.e., a torque direction of a wiper motor is reversed at the transition moment, that is considered as defining a “minus duty position.”
For a control of a variable cam shaft phase in the electric VCT system, the variable and continuous phase control of the cam shaft needs to have a high responsiveness. Further, various control factors need to be controlled in terms of motor control of a motor used in such system. That is, the drive force of the motor in the electric VCT system is subject to various factors, i.e., is subject to the change of a reverse electromotive force in a deceleration process of the motor, a fluctuation of the power source voltage and the like. Therefore, an accurate motor rotation speed control with a sufficient responsiveness is not achievable, based on the duty value calculation of simply using a target rotation speed and an actual rotation speed of the motor.